The present invention relates to a method for preparation of the supersaturated calcium and phosphate ions solution and a method for preparation of the thin film of calcium phosphate crystal on solid surface by using the said solution. Particularly, the lowered temperature and/or the use of suitable buffer system inhibit the nucleation of calcium phosphate in aqueous solution, and thereby highly supersaturated calcium and phosphate ions solution can be prepared. And the thin film of calcium phosphate crystal on solid surface can be prepared with rapidity and good quality of high reactivity and low crystallinity by using the said solution.
Calcium phosphate crystal has been known to have biocompatibility. The apatite crystal is the only type of calcium phosphate crystals in calcified tissues (H.-M. Kim et al., J. Bone Miner. Res. 10, 1589-1601 (1995); U.S. Pat. No. 5,565,502; and U.S. Pat. No. 5,691,397), and it has been used as substitute for bone. Also it has been widely used for enhancing the biocompatibility to tissues by forming the apatite layer on biomaterial surface of metals or as solid particles (R. G. T. Geesink, Clin. Orthop. Relat. Res. 261, 39-58 (1990); and M. G. Dunn and S. H. Maxian, J. Long Term Effect. Med. Implants, 1, 193-203 (1991)).
So far, in order to enhance the biocompatibility by using calcium phosphate crystal, various coating methods have been developed for forming calcium phosphate crystal, especially apatite layer on the surface of substrate such as titanium, etc. The plasma coating has been most widely used. With the plasma coating, the apatite layer is formed and grown at high temperature, therefore it comprises hydroxy apatite which has low bioreactivity due to high crystallinity and large size. Also it has been known that the byproducts such as other phases of calcium phosphate or calcium oxide are formed in addition to the apatite crystal (H.-G. Pfaff et al., Properties of HA-Coatings in xe2x80x9cBioceramicsxe2x80x9d, vol. 6, eds., P. Ducheyne and D. Christiansen, pp. 419-424, Butterworth-Heinemann Ltd. (1993)). It has to be confirmed clearly what negative effect the said byproduct has on biocompatibility. Thus, there has been a demand to develop a method to form apatite crystal layer with properties similar to those of bone apatite crystal, i.e. having favorable bio-reactivity due to low crystallinity and wide reactive surface area due to small size, on substrate surface.
Pure hydroxy apatite crystals consisting of calcium, phosphate and hydroxyl ions are stoichiometric crystals of rods with high crystallinity while biocrystals isolated form bone or calcified cartilage are nonstoichiometric apatites of low crystallinity (Elliott J. C. In Structure and Chemistry of the Apatites and Other Calcium Orthophosphates, Studies in Inorganic Chemistry 18, Amsterdam: Elsevier, pp 111-190 (1994)). Biocrystals are thin plates of extremely small nanocrystals (27.3xc3x9715.8 nm for bone, 103xc3x9768 nm for calcified cartilage; lengthxc3x97width) of extensive specific surface that makes them metabolically active with high surface reactivity (H.-M. Kim et. al., J. Bone Miner. Res. 10:1589-1601 (1995), Posner A. S. et. al. Skeletal Research: An Experimental Approach, Academic Press: New York, pp 167-192 (1979)).
In order to form a crystal layer of which crystallographic properties mimic those of brocrystals, a method to form the crystal layer directly on solid surface at low temperature has been developed by using the solution of calcium and phosphate ions. However, the concentration of ions cannot be kept high because of spontaneous nucleation in supersaturated solution and it was difficult to keep the concentration product [Ca2+]xc3x97[HPO42xe2x88x92] of the solution above 6 mM2 (H. B. Wen et al., J. Biomed. Mater. Res. 41, 227-236 (1998)). In addition, in such low supersaturated concentration, its application to formation of crystal layer on solid surface was very limited to narrow conditions of temperature and solid surfaces. That is, the temperature had to be kept at about 37xc2x0 C., and the coating procedures needed long time, over one month depending on the condition of surface. Temperature controls the crystallographic nature of the crystal layer. In addition, much lowered temperature is needed to attach the temperature-sensitive and bioactive materials with the crystal on the surface. Therefore, in case that the temperature has to be controlled or lowered, the said method can not be employed. And only the charged surface can be employed for the substrate in most of cases.
We, the inventors of the present invention, have developed a method for preparation of the supersaturated calcium and phosphate ions solution and a method for preparation of the thin film of calcium phosphate crystal on solid surface by using the said solution. Particularly, the lowered temperature and/or the use of suitable buffer system inhibited the nucleation of calcium phosphate in aqueous solution, and thereby highly supersaturated calcium and phosphate ions solution could be prepared. And the thin film of calcium phosphate crystal on solid surface could be prepared with rapidity and good quality of high reactivity and low crystallinity by using the said solution. Also the thin film of calcium phosphate crystal prepared according to the present invention has biocompatibility.
The object of the present invention is to provide a method for preparation of the supersaturated solution of calcium phosphate keeping the concentration of calcium and phosphate ions high.
The another object of the present invention is to provide a method for preparation of the thin film of calcium phosphate crystal on solid surface using the said solution with rapidity and good quality of high reactivity and low crystallinity.